What Are Sunspots, and the Solar Cycle

Look at the Sun through a safe solar filter near solar maximum and you'll often see dark freckles scattered across its face. Those are sunspots — and they're the visible heartbeat of the Sun's activity.

Why they look dark

A sunspot is not a hole or a shadow. It's a patch where the Sun's magnetic field is so concentrated and intense that it chokes off the normal churning that brings heat up from below. Starved of that heat, the spot cools to around 3,500 °C — against the 5,500 °C of the surface around it. It's still ferociously bright; it only looks dark by contrast. Lift a sunspot out and set it in the night sky and it would shine like a full Moon.

Why they matter

Where the magnetic field is most tangled, it's most likely to snap — and a snapping field is exactly what produces a solar flare and, sometimes, a coronal mass ejection. So sunspots are the nurseries of space weather. A big, magnetically complex sunspot group is the clearest sign the Sun is about to get interesting; a blank, spotless disk means a quiet stretch.

That's why forecasters number and classify each region. NOAA gives every active region a number (like AR4465), measures its area and spot count, and assigns a magnetic class — and even publishes the daily odds that it will fire a C, M or X flare. You can see all of that on the live sunspots tracker.

The 11-year rhythm

Sunspots don't appear at a steady rate. Their numbers rise and fall over about 11 years:

• Solar minimum — a calm Sun, sometimes spotless for days. Few flares, rare aurora.
• Solar maximum — a crowded, busy disk. Frequent flares, more geomagnetic storms, and aurora reaching far from the poles.

This is the solar cycle, and it's been tracked since the 1700s. Two numbers measure it: the sunspot number (a standardised count, averaged monthly) and the 10.7 cm radio flux (F10.7), a radio-brightness proxy that doesn't even need clear skies to measure. Both appear on our sunspots page as the solar-cycle context.

Why the aurora has been so good lately

When a cycle is near its maximum, everything downstream picks up: more sunspots, more flares, more coronal mass ejections, and aurora pushing to latitudes that normally never see it. If you've noticed northern lights showing up unusually far south in recent years, that's the solar cycle at work — and a glance at the aurora forecast on an active night is the payoff.